This invention relates to an effective and facile method for obtaining [S,S]-ethylenediamine-N,N'-disuccinic acid from an aqueous solution of the salts of such acid and L-aspartic acid.
Ethylenediamine-N,N'-disuccinic acid (EDDS) and its various alkali metal, alkaline earth metal, ammonium and substituted ammonium salts are well recognized by the detergent industry as useful chelating agents in cleaning formulations. (See U.S. Pat. No. 4,704,233, which is incorporated herein by reference as if fully set forth.) These salts and acids are theorized to chelate metals such as iron, manganese, copper and other multivalent metal ions. The metal ions are constituents of certain organic stains or act to stabilize such stains when present in washing solutions. Besides providing for the chelating function, EDDS and its salts are non-phosphorous compounds and, as a result, are environmentally desirable. Even further, EDDS and its salts exhibit biodegradability. The degree of biodegradability depends upon the optical EDDS isomer involved. Of the three optical isomers, [R,R], [R,S] and [S,S], the [S,S] isomer is most easily biodegradable and is thus preferred.
The [S,S] isomer can be synthesized from L-aspartic acid and 1,2-dibromoethane. A particularly attractive route features reacting the aspartic acid as sodium L-aspartate with 1,2-dibromoethane in a basic aqueous medium to yield, in solution, the sodium salts of [S,S] EDDS. See Neal and Rose, Stereospecific Ligands and Their Complexes of Ethylenediamine-disuccinic Acid, Inorganic Chemistry, Vol. 7. (1968), pp. 2405-2412. The Neal and Rose process reacts most of the L-aspartic, with typically less than 60% of the reacted L-aspartic acid being converted to the sodium salt of [S,S] EDDS. The remainder is converted to by-products, such as, oligomers, 2-hydroxyethylamine N-succinic acid and 2-bromoethylamine N-succinic acid, and other heavies.
According to Neal and Rose, the EDDS can be recovered from the solution by slowly acidifying the solution with concentrated hydrochloric acid to obtain a pH of 3.5. The acidification converts the [S,S] EDDS salt to the acid, which acid crystallizes and precipitates from the solution. Fine crystals are said to precipitate out as the pH moves between pH 7 and 3.5. To purify the EDDS precipitate, the precipitate is recovered and redissolved in a NaOH solution followed by reacidification. The cycle is repeated two times. The final precipitate is washed with water to remove HCl and any traces of L-aspartic acid. This procedure is burdened with poor L-aspartic acid utilization, lengthy process time, and high HCl consumption in the purification cycles.
The L-aspartic acid utilization can be substantially improved if, instead of reacting almost all of the L-aspartic acid (as the salt) per Neal and Rose, a smaller portion is reacted, say less than about 60 mole %. In this way, it has been found that most of the L-aspartic acid reacted is converted to [S,S] EDDS (as the salt) and that very little of the reacted L-aspartic acid goes to the production of by-products. However, the low amount of L-aspartic acid reacted presents a significant problem to the facile recovery of [S,S] EDDS. The problem resides in the fact that the substantial amount of unreacted L-aspartic acid salt in the reaction solution will co-precipitate out with the [S,S] EDDS precipitate when the solution is acidified as per Neal and Rose. Due to the large amount of the L-aspartic acid precipitate, the resultant product is not of acceptable purity.
It is, therefore, an object of this invention to provide a simple and efficient method for recovering relatively pure [S,S] EDDS from an aqueous solution containing L-aspartic acid salt and EDDS acid salt.